<!DOCTYPE html>
<html lang="en">
 
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }
        .box1{
            width: 300px;
            height: 200px;
            background-color: aqua;
            float: left;
            margin-bottom: 10px;
            margin-right: 10px;
        }@media screen and (max-width:620px){
            body{
                background-color: brown;
            }
            .box1{
                width: 100%;
            }
        }
    </style>
</head>
<body>
    <div class="box1"></div>
    <div class="box1"></div>
</body>
</html>